Josef Fritzl, the Austrian man accused of imprisoning his daughter for 24 years and fathering her seven children, set the wheels in motion to begin building his secret cell as early as 1978, authorities said Monday.

His daughter Elisabeth was 12 years old at that time.

Police Col. Franz Polzer, who is overseeing the investigation into Fritzl, said the accused applied for permits three decades ago to expand the apartment complex he owned. It is believed Fritzl's expansion plans included the secret rooms, he said.

"This was not built from one day to the next," Polzer said.

Police also said eight doors fitted with sophisticated locks and electronics secured the underground, windowless warren where Fritzl held his daughter captive.

Prosecutors are scheduled to have their first meeting with Fritzl, 73, on Wednesday or Thursday, they told reporters in Amstetten, about 120 kilometres west of Vienna, where the Fritzl home is located.

Investigators said Fritzl, 73, has confessed to locking his daughter Elisabeth in the cell when she was 18 and repeatedly raping her, telling his wife that the girl had run away from home to join a religious cult.

Elisabeth — who is now 42 and until recently hadn't seen daylight since her capture — gave birth to seven of her father's children in the windowless cellar. One child apparently died and was later thrown into an incinerator by Fritzl.

Mental disorder was a trigger: lawyer

Fritzl's lawyer says he has a severe mental disorder and that he is preparing an insanity defence.

"I believe that the trigger was a mental disorder, because I can't imagine that someone has sex with his own daughter without having a mental disorder," Rudolf Mayer said about his client in an interview.

Anyone with that kind of psychological illness "didn't choose" to do what police allege he did, Mayer said.

If experts determine Fritzl can be considered certifiably insane, and if he is convicted, he would go to a psychiatric institution rather than a prison, Mayer said.

Fritzl has not yet been charged, but remains in pretrial detention. His family is receiving psychiatric care and counselling.

The case came to light April 19, when Elisabeth's 19-year-old daughter, Kerstin, became ill and was taken to hospital by Fritzl.

Suspicious of the man's behaviour, authorities compared Fritzl's DNA to a sample taken from Kerstin, and made a televised appeal for the mother to come forward with information about the girl's medical history.

Elisabeth persuaded her father to let her leave the basement for the first time in 24 years so she could meet the doctors. She and her father were detained on April 26, which allowed her to finally reveal her ordeal to authorities.

Kerstin and two siblings, aged 18 and 5, had never been outside the tiny cellar since they were born there. Their other three surviving siblings were raised in the home above them.

With files from the Associated Press